1. Field of the Invention
This invention relates to a process for producing a continuous discharge of insoluble coal products, in a dry powdery form, from a coal deashing system utilizing solvents at temperatures near the critical temperature of the solvent and at elevated pressures.
2. Description of the Prior Art
Various coal processing systems have been developed in the past wherein coal has been treated with one or more liquefaction solvents after which the liquefaction products are processed to separate the resulting insolubles from the soluble coal products.
In U.S. patent application Ser. No. 838,021 filed Sept. 27, 1977 and assigned to the same assignee as the present invention, an improved system for deashing coal liquefaction products, i.e., removing insolubles, is disclosed.
In that system, a feed mixture comprising soluble coal products, solvent and insoluble coal products is separated in a first separation zone maintained at an elevated temperature and pressure into a first heavy fraction comprising insoluble coal products and some of the solvent and a first light fraction. The insoluble coal products comprise the undissolved coal, mineral matter, other solid inorganic particulate matter and other such matter which is insoluble in the solvent solution under the operating conditions of the process.
The first heavy fraction is withdrawn from the first separation zone and the pressure level is reduced at least 100 psig and preferably at least about 500 psig to vaporize the solvent and yield the insoluble coal products in a dry, powdery form referred to as ash concentrate. The powdery ash concentrate composition can be made into aqueous slurries for use as a feed or transported by mechanical means such as, for example, screw feeders or pneumatically to subsequent processing equipment such as for example, gasifiers to recover hydrogen values.
Under certain conditions however, it has been found that the first heavy phase does not yield the desired, dry, powdery ash concentrate composition upon withdrawal and pressure reduction. Rather, the discharged first heavy fraction is in the form of large chunks or a continuous extrusion which plugs the transfer conduit from the first separation zone or the vessel to which the transfer conduit connects, such as for example, a solid-gas separator, such that the process operations must be terminated. Further, the plugs are hard to remove after formation in the transfer lines or vessels and require a tedious and expensive cleanout to restore operations.
Thus, it would be desirable to provide a method by which the plugging problem associated with the withdrawal of the first heavy fraction from the first separation zone could be alleviated.